This invention relates to a hydraulic brake system and, more particularly to a hydraulic brake system for automotive vehicles. Such systems include a master brake cylinder which is connected to an unpressurized reservoir, has a master cylinder piston including a supply valve which opens in the rest position of the piston and is otherwise closed. The supply valve connects a supply bore provided behind the piston seal with a brake line connected to a master cylinder working chamber with which the piston cooperates. The brake line leads to pressure control valves of a brake slip control device, which valves lead to at least one wheel brake. Via a non-return valve the working chamber of the master brake cylinder is connected to a pressure medium source upon the starting of the brake slip control device. The outlet pressure of said pressure medium source lies above the maximum braking pressure achievable by applying the master brake cylinder so that, upon the starting of the brake slip control device, the master cylinder piston is displaced into its rest position. In this position the supply valve is opened until the pressure in the working chamber has dropped to the value corresponding to the actuating force at the brake pedal. The working chamber's front side away from the master cylinder piston is sealed by a second master cylinder piston sealingly arranged within the master cylinder.
In a suggested hydraulic brake system of this type (German Published Patent Application DE-OS No. 36 01 914, the additional pressure source required for brake slip control is in the form of a pump and is directly connected with the brake circuits to be controlled via non-return valves. This, however, implies an increase in the number of lines and the requirement of special non-return valves decoupling the turned-off pressure source from the master cylinder pressure.